Pixel having two semiconductor layers, image sensor including the pixel, and image processing system including the image sensor

ABSTRACT

An image sensor having pixels that include two patterned semiconductor layers. The top patterned semiconductor layer contains the photoelectric elements of pixels having substantially 100% fill-factor. The bottom patterned semiconductor layer contains transistors for detecting, resetting, amplifying and transmitting signals charges received from the photoelectric elements. The top and bottom patterned semiconductor layers may be separated from each other by an interlayer insulating layer that may include metal interconnections for conducting signals between devices formed in the patterned semiconductor layers and from external devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/560,347 filed Sep. 4, 2019, which is a continuation of U.S.application Ser. No. 16/285,686 filed Feb. 26, 2019, which is acontinuation of U.S. application Ser. No. 15/937,632 filed on Mar. 27,2018, issued as U.S. Pat. No. 10,249,677 on Apr. 2, 2019, which is acontinuation of U.S. application Ser. No. 15/047,444 filed on Feb. 18,2016, issued as U.S. Pat. No. 9,954,025 on Apr. 24, 2018, which is acontinuation of U.S. application Ser. No. 13/838,627 filed on Mar. 15,2013, issued as U.S. Pat. No. 9,293,501 on Mar. 22, 2016, which is acontinuation of U.S. application Ser. No. 12/684,163 filed on Jan. 8,2010, issued as U.S. Pat. No. 8,508,012 on Aug. 13, 2013, which is adivisional of U.S. application Ser. No. 11/524,695 filed on Sep. 21,2006, issued as U.S. Pat. No. 7,671,435 on Mar. 2, 2010, which claimspriority under 35 U.S.C. § 119 of Korean Patent Application10-2005-0091293 filed on Sep. 29, 2005, the disclosures of which areincorporated by reference herein in their entireties.

BACKGROUND

The present invention relates to image sensors, and more particularly topixels of image sensors.

Digital cameras are today one of the most popular consumer electronics.The “MegaPixel” capacity and color sensitivity of image sensors iscurrently a limiting factor limiting image quality of images captured bydigital cameras. Image sensors function to transform (visible) lightthat is focused onto the image sensor by optical lenses, into electricalsignals to capture and display images of high quality. Additionally, avery fast “shutter speed” or “film speed” equivalent in digital imagesensors is a strong selling point for high-end digital cameras and canenhance image quality (e.g., reducing blurring due to subject or cameramotion).

A typical image sensor comprises a pixel array that has a plurality(array) of pixels. Each pixel includes a photodiode for generatingsignal charges in response to photons (i.e., light) incident thereon,and an electronic element for transferring and outputting the signalcharges from the photodiode. Depending upon the manner of transferringand outputting signals charges, image sensors are roughly classifiedinto two kinds, i.e., charge-coupled devices (CCD; hereinafter, ‘CCDimage sensors’) and complementary metal-oxide-semiconductor (CMOS) imagesensor (hereinafter, ‘CMOS image sensor’). The CCD image sensor usespluralities of MOS capacitors for accumulating, transferring andoutputting charges. By applying appropriate voltages to the electrodesof the MOS capacitors, signal charges of each pixel are successivelytransferred by way of the MOS capacitors. The CMOS image sensor usespluralities of transistors, by which signal charges generated by thephotodiode are converted into a voltage at each pixel and outputtherefrom.

The CCD image sensors typically have better noise and image quality thanCMOS image sensors, but CMOS image sensors typically have lower productcost and lower power consumption than CCD image sensors. In other words,the CMOS image sensor has the advantages of lower power, singularity ofvoltage and current, compatibility with combined CMOS circuits (e.g.,integrated on the same chip), random access of image data, and lowerproduction cost by employing the standard of CMOS technology. Thus, CMOSimage sensors are gaining market share in various applications such asdigital cameras, smart phones, personal digital assistants (PDA),notebook computers, security cameras, barcode detectors, high-definitiontelevision sets, children's toys, and so on.

FIG. 1 is a plane view of a portion of a pixel array of a conventionalimage sensor. Referring to FIG. 1, pixels are formed around activeregions 13 of a semiconductor substrate, the active regions beingelectrically isolated from each other (like “semiconductor islands”),each of which includes a photodiode and pluralities of transistors. Eachactive region 13 may be sectored into a first active region 13Aincluding the photodiode, and a second active region 13B including theplural transistors. On the second active region 13B are arranged atransfer gate 21, a reset gate 23, a source follower gate 25, and aselection gate 27. The transfer gate 21 is located adjacent to the firstactive region 13A. An impurity region formed in the second active region13A between the transfer and reset gates 21 and 23 serves as a floatingdiffusion region 29 that is electrically connected to the sourcefollower gate 25. An impurity region formed in the second active region13B between the reset and source follower gates 23 and 25 acts as areset diffusion region 31. There is an impurity region 33 in the secondactive region 13B between the source follower and selection gates 25 and27, and an impurity region 35 in the second active region 13B outside ofthe selection gate 27. Each transistor is formed of a gate and theimpurity regions at either side of its gate.

The vertically sectional structure of a representative pixel is nextdescribed with reference to FIG. 2. FIG. 2 is a cross-sectional view ofone pixel taken along section line I-I′ of FIG. 1. Referring to FIG. 2,the photodiode includes an N-type region 17 and a P-type region 19formed in the first active region 13A. The floating diffusion (FD)region 29 is electrically connected to the source follower gate 25through a local interconnection 37.

In the general structure of a conventional pixel, it is necessary toform the photodiode and gates in the same active region 13 and toallocate a part (e.g., 13B) thereof to the transistors. Thus, only aportion, i.e., the first active region 13A of the active region 13 isused for the photodiode. Thus, there is a limitation of the conventionalimage sensor due to a fill factor that represents the area occupied in apixel by the photodiode.

The “fill factor” indicates the size of the light sensitive photodioderelative to the entire pixel and is the fraction of the surface that issensitive to light. A large fill factor is desirable because the largerthe fill factor the more light will be captured by the chip up to themaximum of 100%. This helps improve the Signal-to-Noise Ration (SNR).Because of the extra electronics (e.g., transistors) required in eachpixel the “fill factor” tends to be quite small, especially for ActivePixel Sensors which have more per pixel transistors. To overcome thislimitation, some have proposed an array of microlenses to be placed ontop of the sensors, but this increases the production cost.Additionally, in the conventional image sensor, light incident upon atarget pixel, especially, light incident from a slanted angle, may bereflected by an interconnection of the target pixel or a gate and mayarrive at an adjacent pixel, causing across-talk affect or distortion.

SUMMARY OF THE INVENTION

Various embodiments of the invention provide image sensors having pixelsthat include two patterned semiconductor layers. The top (second)patterned semiconductor layer contains the photoelectric elements (e.g.,photodiodes) of pixels having substantially 100% fill-factor, and maycontain one or more transistors. The bottom (first) patternedsemiconductor layer contains transistors for detecting, resetting,amplifying and transmitting signals charges received from thephotoelectric elements. The top and bottom patterned semiconductorlayers may be separated from each other by an interlayer insulatinglayer that may include metal interconnections for conducting signalsbetween devices formed in the patterned semiconductor layers and fromexternal devices.

Exemplary embodiments of the invention provide a pixel with improvedarchitecture, an image sensor having the pixel, and an image processingsystem employing the image sensor.

Some embodiments of the invention provide an image sensor comprising:first and second semiconductor patterns (i.e., first and secondpatterned semiconductor layers) with an interlayer insulating layerinterposed therebetween; a photoelectric element formed in the secondsemiconductor pattern; and at least one or more transistors formed inthe first semiconductor pattern, electrically connected to thephotoelectric element and sensing signal charges generated from thephotoelectric element.

The photoelectric element may be a photodiode comprising: an N-typeregion formed in the second semiconductor pattern; and a P-type regionformed in the second semiconductor pattern enveloping the N-type region.

The image sensor may further comprise first and second charge collectionregions (electrically connected to each other) formed in the first andsecond semiconductor patterns respectively for collecting the signalcharges generated in the photoelectric element. The one or moretransistors may be operationally coupled with the first and secondcharge collection regions. The one or more transistors may comprise afirst transistor for sensing and a second transistor for resettingsignal charges, of the first and second charge collection regions. Thefirst and second transistors are connected with each other by a commondrain. A gate of the first transistor is electrically connected to thefirst and second charge collection regions. The first charge collectionregion acts as a source of the second transistor.

The image sensor may further comprise a transfer gate in the secondsemiconductor pattern (e.g., beneath the photodiode) for transferringthe signal charges to the first and second charge collection region fromthe photoelectric element.

The photoelectric element may be a diode comprising: an N-type regionformed in the second semiconductor pattern; and a P-type region, formedin the second semiconductor pattern, enveloping the N-type region. Thesecond charge collection region may be formed in the P-type regionoutside of the transfer gate, being spaced apart (separated) from theN-type region.

The image sensor may further comprise a color filter formed on (e.g.,directly on) the second semiconductor pattern. The photoelectric elementmay directly contact the second semiconductor pattern. The photoelectricelement may interpose a buffer layer with the color filter.

Other embodiments of the present invention provide an image sensorcomprising: first and second semiconductor patterns (i.e., first andsecond patterned semiconductor layers) with an interlayer insulatinglayer interposed therebetween; a photoelectric element (e.g.,photodiode) in the second semiconductor pattern; a transfer gate belowthe photoelectric element in the second semiconductor pattern fortransferring signal charges from the photoelectric element to a secondcharge collection region in the second semiconductor pattern; a firstcharge collection region in the first semiconductor pattern andelectrically connected to the second charge collection region; a sourcefollower gate in the first semiconductor pattern and electricallyconnected to the first and second charge collection regions; firstsource and drain regions in the first semiconductor pattern at eitherside of the source follower gate; and a reset gate in the firstsemiconductor pattern between the first drain region and the firstcharge collection region.

The photoelectric element may be a diode comprising: an N-type regionformed in the second semiconductor pattern; and a P-type region formedin the second semiconductor pattern so as to envelop the N-type region.The first charge collection region is formed in the second semiconductorpattern outside of the transfer gate and spaced apart from the N-typeregion.

The interlayer insulating layer may comprise first and second interlayerinsulating layers. The first charge collection region is electricallyconnected to the second charge collection region by: a first contactplug penetrating the first interlayer insulating layer and electricallyconnected to the second charge collection region; a local conductivepattern formed on the first interlayer insulating layer and electricallyconnected to the first contact plug; and a second contact plugpenetrating the second interlayer insulating layer and electricallyconnecting the first charge collection region with the local conductivepattern.

The local conductive pattern is electrically connected with a thirdcontact plug that penetrates the first interlayer insulating layer andis electrically connected to the source follower gate.

The interlayer insulating layer may include metal interconnections forapplying bias voltages to the transfer and reset gates.

The image sensor may further comprise: a second drain region formed inthe first semiconductor pattern; and a selection gate formed in thefirst semiconductor pattern between the second drain region and thefirst source region. The interlayer insulating layer may include a metalinterconnection for applying a bias voltage to the selection gate.

Further other embodiments of the present invention provide an imagesensor comprising pixels: each pixel includes a photoelectric elementformed in a second semiconductor pattern (i.e., in a first patternedsemiconductor layer); and at least one or more transistors formed in afirst semiconductor pattern (i.e., in a second patterned semiconductorlayer) and operationally coupled to the photoelectric element, the firstsemiconductor pattern being spaced apart from (separated from) orelectrically insulated from the second semiconductor pattern.

Still further embodiments of the present invention provide an imageprocessing system comprising: a processor; and an image sensoroperationally coupled with the processor. The image sensor comprises aplurality of pixels. Each pixel comprises: a photoelectric elementformed in a second semiconductor pattern (i.e., in a second patternedsemiconductor layer); and at least one or more transistors formed in afirst semiconductor pattern (i.e., in a first patterned semiconductorlayer) and operationally coupled to the photoelectric element, the firstsemiconductor being spaced apart from (separated from) or electricallyinsulated from the second semiconductor.

A further understanding of the nature and advantages of the inventionherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

Exemplary embodiments of the present invention will be described belowin more detail with reference to the accompanying drawings. The presentinvention may, however, be embodied in different forms and should not beconstrued as limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art.

In the figures, the size of layers and regions are exaggerated forclarity of illustration. It will also be understood that when a layer(or film) is referred to as being ‘on’ another layer or substrate, itcan be directly on the other layer or substrate, or intervening layersmay also be present. Further, it will be understood that when a layer isreferred to as being ‘under’ another layer, it can be directly under,and one or more intervening layers may also be present. In addition, itwill also be understood that when a layer is referred to as being‘between’ two layers, it can be the only layer between the two layers,or one or more intervening layers may also be present. Like referencenumerals refer to like elements throughout. It will be understood that,although the terms for first, second, third, etc. may be used herein todescribe various elements such as region, film, layer, etc., theseelements should not be limited by these terms. These terms are only usedto distinguish one element from another. For example, a first layercould be termed a second layer, and, similarly, a second layer could betermed a first layer without depart from the teaching of the disclosure.

In this description, for example, a sentence ‘first element isoperationally coupled with second element’ means that a specificterminal or node of the first element is connected to a specificterminal or node of the second element directly or indirectly through aconductive region. For example, the first or/and second elements are notrestrictive to embodied descriptions herein in terminology, but may beused for illustrating transistors, impurity regions, or various kinds ofconductive interconnections.

In the description, for example, a sentence ‘transistor is operationallycoupled with photodiode’ may represent that an impurity region of aphotodiode acts as a source or drain region of a transistor, an impurityregion of a photodiode is electrically connected to the source or drainregion of the transistor, or an impurity region of a photodiode iselectrically connected to a gate of the transistor.

In the description, for example, a sentence ‘first and secondtransistors are operationally coupled with each other’ may be understoodsuch that a voltage applied to a gate of the first transistor istransferred to a terminal or node of the second transistor, e.g., agate, a source region, or a drain region, directly or indirectly througha third transistor or a conductor such a metal interconnection.Alternatively, it may indicate that a terminal or node of the firsttransistor is electrically connected with a terminal or node of thesecond transistor.

In the specification herein, ‘semiconductor substrate’, ‘semiconductorlayer’, ‘semiconductor pattern’, or ‘substrate’ will be used torepresent a structure based on a semiconductor. This structure based onsemiconductor may be comprised of a silicon layer, asilicon-on-insulator (SOI) where a silicon layer is disposed on aninsulation layer, a doped or undoped silicon layer, an epitaxial layerformed by a technique of epitaxial growth, or another semiconductorconstruction.

While the exemplary embodiments of the present invention are applicableto all kinds of image sensors, the embodied description herein will beillustrative a CMOS image sensor as an example for appreciation of thefeatures by the invention.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures are included to provide a further understandingof the invention, and are incorporated in and constitute a part of thisspecification. The drawings illustrate exemplary embodiments of theinvention and, together with the description, serve to explainprinciples of the present invention. In the figures:

FIG. 1 is a plane view of a portion of a pixel array of a typical imagesensor;

FIG. 2 is a cross-sectional view of a pixel taken along section lineI-I′ of FIG. 1;

FIG. 3 is a perspective view of a pixel in an image sensor in accordancewith an embodiment of the present invention;

FIG. 4 is a plane view of a portion of a pixel array of the image sensorof FIG. 3;

FIG. 5 is a cross-sectional view of a pixel taken along section lineII-II′ of FIG. 4;

FIG. 6 is a block diagram of the image sensor including the pixels ofFIGS. 3, 4 and 5; and

FIG. 7 is a block diagram of a system with the image sensor shown inFIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 is a perspective view of a pixel in an image sensor in accordancewith an embodiment of the invention. Referring to FIG. 3, a pixel 101according to the embodiment by the invention comprises a photodiode 115,and transfer, reset, source follower, and selection transistorsoperationally coupled with the photodiode 115. According to thisembodiment of the invention, the components of the pixel 101 arearranged in separate first and second semiconductor patterns 111 and 113that are spaced apart from each other. In this exemplary embodiment ofthe invention, the photodiode 115 and the transfer transistor aredisposed in the second semiconductor pattern 113, while the reset,source follower, and selection transistors are placed in the firstsemiconductor pattern 111. The gate 217 of the transfer transistor isdisposed under the photodiode 115. Thus, the fill factor of the pixel isnot affected by the presence or size of the transfer, reset, sourcefollower, and selection transistors therein.

According to this embodiment, as the second semiconductor pattern 113 isused entirely for the photodiode 115, it is possible to achieve the fillfactor of substantially 100%.

In this embodiment, since the first semiconductor pattern 111 includingthe transistors does not affect the fill factor, the first semiconductorpattern 111 may be formed in the same size as the second semiconductorpattern 113. Thus, it is possible to improve the noise characteristic of1/f and the light-gathering performance (e.g., speed) of the pixel. Inaddition, the charge collection regions 411_1 and 411_2, may be enlargedto extend the dynamic range thereof.

The photodiode 115 includes a first conductive region (e.g., an N-typeregion) 113 n formed in the second semiconductor pattern 113 and asecond conductive region (e.g., a P-type region) 113 p enveloping thefirst conductive region 113 n. In this embodiment, electron-hole pairsas signal charges are generated in response to photons incident upon thesecond semiconductor pattern 113, and electrons are accumulated in theN-type region 113 n. Since the N-type region 113 n is entirely enclosed(enveloped) by the P-type region 113 p, the leakage of electrons out ofthe N-type region 113 n is minimized.

The transfer gate 217 is disposed under a gate insulation layer 317adjacent to the N-type region 113 n, interposed between the gateinsulation layer 317 and the N-type region 113 n. It can be seen thatthe transfer transistor includes the transfer gate 217, and that thesecond charge collection region 411_2 and the N-type region 113 n arepositioned at either side of the transfer gate 217. If a bias voltage isapplied to turn ON the transfer gate 217, the charges (e.g., electrons)accumulated in the N-type region 113 n are transferred to the secondcharge collection region 411_2 functioning as a floating diffusionregion. The second charge collection region 411_2 is formed in thesecond semiconductor pattern 113 outside of the transfer gate 217, beingdoped with N-type impurities.

On the first semiconductor pattern 111, a reset gate 211, a sourcefollower gate 213, and a selection gate 215 are formed on gateinsulation layers 311, 313, and 315 formed on the first semiconductorpattern 111. Impurity regions in the first semiconductor pattern 111,are source/drain regions. The gate and the impurity regions at eitherside of a gate constitute a transistor. For instance, a reset transistorincludes the reset gate 211 and the impurity regions 411_1 and 413 ateither side of the reset gate 211. A source follower transistor includesthe source follower gate 213 and the impurity regions 413 and 415 ateither side of the source follower gate 213. A selection transistorincludes the selection gate 215 and the impurity regions 415 and 417 ateither side of the selection gate 215. A VDD voltage (from a powersupply not shown) is applied to the impurity region 413 between thereset and source follower gates 211 and 213.

The impurity region 411_1 of the reset transistor is electricallyconnected to the second charge collection region 411_2, acting as afloating diffusion region (similar to the second charge collectionregion 411_2). In other words, the impurity region 411_1 of the resettransistor accumulates charges transferred from the photodiode 115(which hereinafter will be referred to as the first charge collectionregion in recognition of the fact that signal charges are firstaccumulated therein). When a bias voltage is applied to the reset gate211, a conductive channel is formed under the reset gate 211 in thefirst semiconductor pattern 111 and signal charges remaining in thefirst and second charge collection regions 411_1 and 411_2 flow into apower source (not shown) connected to the impurity region 413 of thereset transistor. The pixel is thereby initialized.

The source follower gate 213 (of the source follower transistor) iselectrically connected to the first and second charge collection regions411_1 and 411_2. The first and second charge collection regions, 411_1and 411_2, and the source follower gate 213 are electrically connectedwith each other by way of a local conductive pattern 611 and contactplugs 511, 513, and 711, forming a common node. Thus, a signal voltage,corresponding to (e.g., proportionate with) the amount of signal chargesaccumulated in the first and second charge collection regions 411_1 and411_2, appears at the impurity region 415 of the source followertransistor. When a bias voltage is applied to the selection gate 215 ofthe selection transistor, the signal voltage (at the impurity region415) is transferred to an output terminal of the selection transistor,i.e., to the impurity region 417. The signal transferred to the outputterminal 417 of the selection transistor is detected and processed by aperipheral circuit (not shown). The signal processing operationsperformed by peripheral circuits are well known to persons skilled inthe art and will be described with reference to FIG. 6 below.

The first semiconductor pattern 111, including the reset, sourcefollower, and selection transistors, may be a P-type siliconsemiconductor substrate. The transistors formed in the firstsemiconductor pattern 111 may be formed by, for example, depositing andpatterning each of a gate insulation layer and a conductive layer, andimplanting ionic impurities to form the impurity regions. A conductivelayer for the gate is not restricted to this exemplary embodiment andmay be formed of other materials or in other structures, for example,polysilicon, or a multi-layer of polysilicon and silicide. When thefirst semiconductor pattern 111 is a P-type, N-type ionic impurities areinjected to form source/drain regions of the transistors.

An interlayer insulating layer (911, see FIG. 5, not shown in FIG. 3) isinterposed between the first and second semiconductor patterns 111 and113, as will be detailed with reference to FIG. 5.

The transfer gate 217 may be formed by depositing a conductive layer onthe interlayer insulating layer (911 shown in FIG. 5) and patterning theconductive layer. Patterning of the conductive layer may be conducted bya photolithography process. The gate insulation layer 317 covering(insulating) the transfer gate 217 may be formed by a film depositiontechnique. The second semiconductor pattern 113 disposed on the secondinterlayer insulating layer (813 shown in FIG. 5), covering the transfergate 217 and the gate insulation layer 317 thereon, may be formed bymeans of a film deposition technique such as chemical vapor deposition(CVD) or plasma-enhanced CVD, or epitaxial growth, the methods offormation not being restricted to those examples.

The photodiode 115 may be formed by conducting ion implantation into thesecond semiconductor pattern 113. For instance, the photodiode 115 maybe produced (after forming the second semiconductor pattern 113 dopedwith P-type impurities), by implanting ionic impurities to form theN-type region 113 n and implanting ionic impurities to form the topP-type region 113 p. According to this embodiment, the photodiode 113constitutes a vertical PNP structure, thereby avoiding the effect ofimage lag.

The second charge collection region 412_2 acting as the floatingdiffusion region may be formed by implanting ionic impurities into thesecond semiconductor pattern 113 and using the transfer gate 217 as anion injection mask.

The steps of ion implantation for the photodiode 115 and the secondcharge collection region 411_2 may proceed in an appropriate order.

The contact plugs, 511, 513, and 517, may be formed by patterning theinterlayer insulating layer(s) (811, 813 as shown in FIG. 5) to formcontact holes and then filling the contact holes with a conductivematerial. The local conductive pattern 611 is may be formed bydepositing and patterning a conductive layer (upon interlayer insulatinglayer 811). The contact plugs 511 and 513 connected each to the firstcharge collection region 411_1 and the source follower gate 213 may beformed at the same time through the interlayer insulating layer 811.

Interconnections not shown are disposed between the first and secondsemiconductor patterns 111 and 113 in order to apply bias voltages tothe reset gate 211, the selection gate 215, and the transfer gate 217.The interconnections not shown may be formed while forming the localconductive pattern 611.

While forming the contact plugs 511 and 513 connected to the firstcharge collection region 411_1 and the source follower gate 213respectively, a contact plug (not shown) connected to the selection gate215 may be formed at the same time. And, at the same time that thecontact plug 711 is formed to connect the local conductive pattern 611with the second charge collection region 411_2, a contact plug (notshown) for connecting the transfer gate 217 with an interconnection thatconducts a bias voltage to the transfer gate 217 may be also be formed.

A processing sequence for forming the contact plugs, theinterconnections, and the local conductive pattern may be varied inalternative modes. According to this embodiment, as the interconnectionsfor applying bias voltage to the plural gates are formed under thephotodiode 115, it is possible to secure misalignment margins for theinterconnections, providing flexibility in arranging theinterconnections.

In this exemplary embodiment, a color filter may be disposed over thephotodiode, so to minimizes optical and electrical cross-talk therein.In addition, since the photodiode is very close to or contacts with thecolor filter and has a large fill-factor, it may not require amicro-lens for condensing light.

A light shielding pattern can be formed under the photodiode withoutdegrading the fill factor of the pixel, and it is possible to minimizeelectrical interference more effectively.

The photodiode may be formed after completing almost all of metalinterconnections. Thus, since there is no metal contact on thephotodiode, a dark level thereof can be minimized.

FIG. 4 is a plane view illustrating part of the pixel array of the imagesensor of FIG. 3, and FIG. 5 is a cross-sectional view of a pixel in thearray of FIG. 4 taken along section line II-II′ in FIG. 4.

Referring to FIG. 4, the first semiconductor pattern 111, including thereset gate 211, the source follower gate 213, and the selection gate215, is located under the second semiconductor pattern 113 and isentirely covered by the second semiconductor pattern 113. Therefore, thedimensions of the pixel are determined by the size of the secondsemiconductor pattern 113 including the photodiode 115. The secondsemiconductor pattern 113 can be used entirely as the photodiode 115. Asillustrated in FIG. 4, a gate width (or the width of the active region)can be enlarged in by extension of the first semiconductor pattern 111along the y axis, by which enlarged width the performance of transistorbecomes improved. The width of the first semiconductor pattern 111 maybe extended so as to make the first semiconductor pattern 111 the samesize as the second semiconductor pattern 113. Further, since the firstsemiconductor pattern 111 is disposed under the second semiconductorpattern 113 including the photodiode 115, it is permissible to variouslymodify the configuration of the first semiconductor pattern 111 withoutreducing the fill factor. For instance, upon altering the configurationof the first semiconductor pattern 111 in various ways, it is possibleto design channel patterns suitable for the optimum performance of thetransistors without reducing the fill factor.

In addition, because the transfer gate 127 is disposed under thephotodiode 115, the gate length of the transfer gate 127 may bevariously designed for the optimum transfer efficiency.

A cross-section of the pixel of FIG. 3 and FIG. 4 can be seen from FIG.5. In FIG. 5, the reference numerals 811 and 813 denote first and secondinterlayer insulating layers and are collectively referred to by thereference numeral 911. The reference numeral 1111 indicates a colorfilter. The first and second interlayer insulating layers 811 and 813may be formed of, for example, borophospho-silicate glass (BPSG) dopedwith boron (B) and phosphorous (P), boro-silicate glass (BSG) doped withboron, phosphor-silicate glass (PSG) doped with phosphorous, undopedsilicate glass (USG), or vapor-deposited silicon oxide. The color filtermay be formed by a conventional process.

Referring to FIG. 5, the color filter 1111 is arranged close or indirect contact with the top of the photodiode 115. In the conventionalimage sensor shown in FIG. 1 or 2, because various kinds ofinterconnections are arranged over the photodiode, the color filter isinevitably spaced apart from (above) the photodiode. And theconventional image sensor uses microlenses to raise the efficiency oflight sensing. Further, due to the distance between the color filter andthe photodiode in the conventional image sensor, the light passingthrough the color filter may arrive at an adjacent pixel as well as atarget pixel. In exemplary embodiments of the present invention, sincethe color filter 1111 is disposed close to or in direct contact with thephotodiode 115, light passing through the color filter is entirelyincident on the photodiode 115 in substance. Moreover, because, inexemplary embodiments of the invention, the photodiode is locateddirectly under the color filter, a microlens need not be formed in thepixel.

FIG. 6 is a block diagram of the image sensor 2080 including the pixelof FIGS. 3, 4 and 5. Referring to FIG. 6, the pixel array 2000 includesa plurality of pixels arranged in a matrix. The matrix of the pixelarray 2000 includes rows and columns of pixels. A row driver 2100selects a specific row of pixels in the pixel array 2000 in response toan output of a row decoder 2200, and a column driver 2600 selects aspecific column of pixels in the pixel array 2000 in response to anoutput of a column decoder 2700. The CMOS image sensor is controlled bya controller 2500. The controller 2500 controls the row decoder 2200,the row driver 2100, the column decoder 2700, and the column driver2600.

An output signals from each of the pixels include a pixel reset signalVrst and a pixel image signal Vsig. The pixel reset signal Vrstcorresponds with the potential of the charge collection region when thepixel is in a reset state. The pixel image signal Vsig corresponds withthe potential of the charge collection region after signal chargesgenerated from an image have been transferred to the charge collectionregion. The pixel reset signal Vrst and the pixel image signal Vsig areread out by a sampling/holding circuit 2610. An amplifier (AMP) 2620generates a difference signal Vrst−Vsig from the reset and image signalsVrst and Vsig. The difference signal is transformed into a digitalsignal by an analog-digital converter (ADC) 2750. An image processor2800 generates a digital image from the digitized differential signals.The image sensor 2080 may be included in a semiconductor chip (e.g., awafer 3000).

FIG. 7 is a block diagram of a processor-based system 4000 including theimage sensor of FIG. 6. The processor-based system 400 may be, anydigital circuit that may employ the image sensor 4080. Theprocessor-based system is not limited hereto, but may be a computersystem, a camera system, a cell-phone, a scanner, a videophone, asurveillance system, a machine vision system, a vehicle navigationsystem, an automatic focus system, a star tracking system, a motiondetection system, an image stabilization system, a data compressionsystem, or other system compatible with an image sensor.

The system 4000 includes a processor (e.g., central processor unit, CPU)4020 communicating with plural devices or peripherals via a bus 4040.The devices (peripherals) coupled to bus 4040, are e.g., an input/outputunit 4060 and the image sensor 4080, provide the system 4000 withinput/output communication. The devices coupled to bus 4040, include atleast one peripheral memories, such as a RAM 4100, a hard disc driver(HDD) 4120, a floppy disc driver (FDD) 4140, and a compact disc (CD)driver 4160. The image sensor 4080 receives control signals as data fromthe processor 4020 or from another device of the system 4000. The imagesensor 4080 provides the processor 402 with a data signal defining animage on basis of the received control signals or data, and theprocessor 4020 processes the signal supplied from the image sensor 4080.

Accordingly in exemplary embodiments of the invention, a fill factor ofsubstantially 100% can be attained because the second semiconductorpattern 113 is used entirely for the photodiode.

In exemplary embodiments of the invention, (see FIGS. 3, 4, 5) the firstsemiconductor pattern 111 including the transistors does not affect thefill factor, and may be formed at the same size (area) as the secondsemiconductor pattern 113. Thus, it is possible to improve the noisecharacteristic of 1/f and also the performance of light-sensingoperations. In addition, exemplary embodiments of the inventionfacilitate enlargement of charge collection regions 411_1 and 411_2, toextend dynamic range.

In exemplary embodiments of the invention, the photodiode 113constitutes a vertical PNP structure that avoids an effect of image lag.Image lag occurs in conventional image sensors when traces of a previousframe (image) remain in future frames, i.e. when the pixel is not fullyreset.

In exemplary embodiments of the invention, since the first semiconductorpattern 111 is disposed under the second semiconductor pattern 113including the photodiode 115, it is permissible to variously modify theconfiguration of the first semiconductor pattern 111 without reducingthe fill factor. For instance, by altering the configuration of thefirst semiconductor pattern 111 in various forms, it is possible todesign channel patterns suitable for the optimum performance of thetransistors.

In exemplary embodiments of the invention, since the color filter isdisposed close to or directly contacting the photodiode, light passingthrough the color filter is entirely incident upon the targetphotodiode.

In exemplary embodiments of the invention, since the photodiode islocated directly under the color filter, the microlens provided inconventional image sensors may be omitted entirely.

In exemplary embodiments of the invention, forming a light shieldingpattern under the photodiode does not degrade the fill factor of thepixel, and it is possible to minimize electrical interference moreeffectively.

In this embodiment, the photodiode is formed after formation of almostall of the metal interconnections. Thus, since there is no metal contacton the photodiode, it is able to minimize a dark level thereof.

The above-disclosed subject matter is to be considered illustrative, andnot limiting, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments, which fall withinthe true spirit and scope of the present invention. Thus, to the maximumextent allowed by law, the scope of the present invention is to bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing detailed description.

What is claimed is:
 1. An image sensor comprising: a first semiconductorsubstrate including a first surface and a second surface, at least onetransistor formed on the first surface of the first semiconductorsubstrate; a first insulating layer stacked on the at least onetransistor; a second semiconductor substrate having a first surface anda second surface, a photodiode formed in the second semiconductorsubstrate; a transfer gate pattern and a floating diffusion of atransfer transistor formed on the first surface of the secondsemiconductor substrate; a second insulating layer disposed between thefirst surface of the second semiconductor substrate and the firstinsulating layer; wherein the first surface of the first semiconductorsubstrate and the first surface of the second semiconductor substratefaces each other, and the first and second insulating layers areinterposed between the first and second semiconductor substrates.
 2. Theimage sensor of claim 1, wherein the at least one transistor includes asource follower transistor connected to the floating diffusion throughat least one contact plug penetrating the first and second insulatinglayers.
 3. The image sensor of claim 2, wherein the at least one contactplug includes a first contact plug formed in the first insulating layerand a second contact plug formed in the second insulating layer.
 4. Theimage sensor of claim 3, further comprising: a conductive pattern formedbetween the first contact plug and the second contact plug, wherein theconductive pattern is disposed at an interface between the firstinsulating layer and the second insulating layer.
 5. The image sensor ofclaim 3, wherein the source follower transistor and the first and secondcontact plugs are disposed below the photodiode in a sectional view. 6.The image sensor of claim 4, wherein the source follower transistor, thefirst contact plug, the second contact plug, and the conductive patternare disposed below the photodiode in a sectional view.
 7. The imagesensor of claim 4, wherein the at least one transistor further includesa reset transistor connected to the floating diffusion through a thirdcontact plug.
 8. The image sensor of claim 7, wherein the first contactplug and the third contact plug are electrically connected to each otherand formed in the first insulating layer, wherein the first contact plugand the third contact plug are formed simultaneously.
 9. The imagesensor of claim 2, wherein the at least one transistor further includesa selection transistor connected to the source follower transistor andoutputs a signal voltage originated from the photodiode
 10. The imagesensor of claim 9, wherein the first semiconductor substrate furtherincludes a peripheral circuit to process the signal voltage.
 11. Animage sensor comprising: a first semiconductor substrate including afirst surface and a second surface, at least one transistor formed onthe first surface of the first semiconductor substrate; a firstinsulating layer stacked on the at least one transistor; a secondsemiconductor substrate having a first surface and a second surface, aphotodiode formed in the second semiconductor substrate; a transfer gatepattern and a floating diffusion of a transfer transistor formed on thefirst surface of the second semiconductor substrate; a second insulatinglayer disposed between the first surface of the second semiconductorsubstrate and the first insulating layer; wherein the first surface ofthe first semiconductor substrate and the first surface of the secondsemiconductor substrate faces each other, and the first and secondinsulating layers are interposed between the first and secondsemiconductor substrates, wherein the transfer gate pattern of thetransfer transistor is disposed below the photodiode in a sectionalview.
 12. The image sensor of claim 11, wherein the floating diffusionof the transfer transistor is also disposed below the photodiode at aside of the transfer gate pattern.
 13. The image sensor of claim 11,wherein the transfer gate is formed in a recess region of the firstsurface of the second semiconductor substrate.
 14. The image sensor ofclaim 11, wherein the at least one transistor includes a source followertransistor connected to the floating diffusion through at least onecontact plug penetrating the first and second insulating layers.
 15. Theimage sensor of claim 14, wherein the at least one contact plug includesa first contact plug formed in the first insulating layer and a secondcontact plug formed in the second insulating layer.
 16. An image sensorcomprising: a first semiconductor substrate including a first surfaceand a second surface, at least one transistor formed on the firstsurface of the first semiconductor substrate; a first insulating layerstacked on the at least one transistor; a second semiconductor substratehaving a first surface and a second surface, a photodiode formed in thesecond semiconductor substrate; a transfer gate pattern and a floatingdiffusion of a transfer transistor formed on the first surface of thesecond semiconductor substrate; a second insulating layer disposedbetween the first surface of the second semiconductor substrate and thefirst insulating layer; wherein the first surface of the firstsemiconductor substrate and the first surface of the secondsemiconductor substrate faces each other, and the first and secondinsulating layers are interposed between the first and secondsemiconductor substrates, wherein at least a part of the transfer gatepattern of the transfer transistor is disposed in the secondsemiconductor substrate.
 17. The image sensor of claim 16, wherein thefloating diffusion of the transfer transistor is also disposed below thephotodiode at a side of the transfer gate pattern.
 18. The image sensorof claim 16, wherein the at least one transistor includes a sourcefollower transistor connected to the floating diffusion through a firstcontact plug and a second contact plug, wherein the first contact plugis in the first insulating layer and the second contact plug is in thesecond insulating layer.
 19. The image sensor of claim 18, furthercomprising: a conductive pattern formed between the first contact plugand the second contact plug, wherein the conductive pattern is disposedat an interface between the first insulating layer and the secondinsulating layer.
 20. The image sensor of claim 3, wherein the sourcefollower transistor and the first and second contact plugs are disposedbelow the photodiode.